The present invention relates to a recording device using a semiconductor laser in which an image such as a picture having halftones is reproduced with several tens of levels of halftones.
In order to record a halftone image, a laser beam may be intensity-modulated according (1) a first technique using an ultrasonic light modulator, (2) a second technique in which the discharge current of a gas laser is varied, or (3) a third technique in which the current flow to a semiconductor laser is varied.
The first technique is disadvantageous in that the provision of an expensive ultrasonic light modulator is necessary and it is necessary to provide a mechanism for making fine position adjustments of the modulator so that the modulator is operated in conformance with a Bragg angle with the result that the device according to the first technique has a high manufacturing cost and intricate construction.
The second technique is also disadvantageous in that, for gas laser discharge current modulation, the modulation frequency must be in a low frequency range of several hundred Hz and that the variations of the discharge current reduce the service life of the laser tube.
The third technique has a drawback that, as the semiconductor laser has an optical output-current characteristic as shown in FIG. 1, the optical output changes greatly merely by slightly changing the current. Accordingly, it is considerably difficult to modulate the optical output through more than several tens of levels by changing the current.
An object of this invention is then to provide a laser type recording device which can record a halftone image of more than several tens of levels.
More specifically, it is an object of the invention to provide a laser type recording device having several tens or several hundreds of light modulation levels.
A further object of the invention is to provide a laser type recording device which is capable of recording an image rich in halftones.